An optical fiber is a flexible transparent fiber typically formed from silica, plastic, or a polymer. Optical fibers are used in fiber optic communications, where they convey optical signals between ends of the optical fibers. Optical fibers include a core material that conveys the optical signals, which is surrounded by a transparent cladding. The transparent cladding has a lower index of refraction than the core, which keeps the optical signals mostly confined to the core due to total internal reflection. However, some of the optical signals in the core scatter from the core and are absorbed by a protective material that surrounds the optical fiber.
While silica is mostly transparent to optical wavelengths, some attenuation still occurs. This attenuation is also dependent on the wavelength of the optical signal. Thus, the optical power of the optical signal along the optical fiber decreases as a distance from the transmitting end of the optical fiber increases. If the optical power at the receiving end of the optical fiber is too low, then the optical signal may not be recovered correctly due to bit errors arising from a low signal-to-noise ratio.
To solve this problem, the optical signal may be amplified using an Erbium Doped Fiber Amplifier (EDFA). EDFAs utilize an active optical fiber that is placed in-line somewhere along the length of the optical fiber. The active optical fiber has an Erbium doped core, which is pumped by a laser diode from one or both ends of the optical fiber. The pump light from the laser diode(s) excite the Erbium ions in the doped core, which amplify the optical signals. The remote pumping of an optical amplifier is referred to as a Remote Optically Pumped Amplifier (ROPA).
Typically, the pump light has a wavelength of about 980 nanometers (nm), while the optical signal through a core of an optical fiber has a wavelength of about 1550 nm. 1550 nm is typically used for optical fibers that use silica as a core material, because the attenuation in silica at 1550 nm is lower than at other wavelengths.
One problem with ROPAs is that the attenuation coefficient of silica at 850 nm is much higher than the attenuation coefficient at 1550 nm. When the pump light is conveyed from an end of the optical fiber, the ROPAs have to be relatively close to the end that is providing the pump light in order to maintain a sufficient pump power at the active optical fiber. This limits how far from the end of the optical fiber that the ROPA can be placed, which can limit the length of the optical fiber before the optical signal degrades past the point of recovery. It is therefore desirable to provide other means of amplifying optical signals in an optical fiber that can extend the effective length of the optical fiber before amplification is required.